Génération de seconde harmonique

Résumé

vignette|Niveaux d'énergie impliqués dans la création de SHG La génération de seconde harmonique (GSH ou SHG en anglais, également appelé doublage de fréquence) est un phénomène d'optique non linéaire dans lequel des photons interagissant avec un matériau non linéaire sont combinés pour former de nouveaux photons avec le double de l'énergie, donc avec le double de la fréquence ou la moitié de la longueur d'onde des photons initiaux. La génération de seconde harmonique, en tant qu'effet optique non linéaire d'ordre pair, n'est autorisée que dans les milieux sans centre d'inversion . C'est un cas particulier de génération de somme de fréquence (2 photons), et plus généralement de génération d'harmonique. La (un cas particulier de conversion spontanée basse) est son processus inverse, où un seul photon conduit à une paire de photons ayant chacun la moitié de l'énergie incidente (et donc la moitié de la fréquence), et se produit en parallèle de la GSH, avec une probabilité plus faible ce

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https://fr.wikipedia.org/wiki/G%C3%A9n%C3%A9ration_de_seconde_harmonique

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Integrated Gallium Phosphide Photonics

Katharina Schmeing

The integration of new materials mediating light-matter interaction in nanoscale devices is a persistent goal in nanophotonics. One of these materials is Gallium phosphide, which offers an attractive combination of a high refractive index (n=3.05 at a wavelength of 1550 nm) and a large bandgap (Eg =2.26 eV), enabling photonic devices with strongly confined light fields, not suffering from heating due to two-photon absorption at telecommunication wavelengths. Furthermore, due to its non-centrosymmetric crystal structure, it has a non-vanishing second-order susceptibility and is piezoelectric. Related to its large refractive index is a high third-order susceptibility. Prior to this work the use of GaP for photonic devices was limited to individual non-integrated components, as GaP was not available on a substrate with substantially lower refractive index equivalent to SOI-wafers for silicon. In this work a process was developed that allows the integration of GaP devices onto SiO2. It exploits direct wafer bonding of a GaP/AlxGa1-xP/GaP heterostructure onto a SiO2-on-Si wafer. After substrate removal, photonic devices are patterned by dry-etching in the top GaP device layer. The GaP devices investigated here are used to explore nonlinear optics and optomechanics. In the area of nonlinear optics, second- and third-harmonic generation are observed. The Kerr coefficient is experimentally estimated as n2[1550nm] = 1.2(5)x10^17m^2/W, for the first time in a precision measurement at telecommunication wavelengths. Four-wave mixing is used for broadband frequency comb generation, where a power threshold as low as 3 mW is obtained. The combination of four-wave mixing and second-harmonic generation leads to frequency-doubled combs. The optomechanical properties of GaP one-dimensional photonic crystal cavities are optimized by simulations and fabricated devices are characterized. Optical quality factors of Qo>10^5 and optomechanical coupling strengths of g0/2pi=400 kHz are measured. Dynamical backaction in the form of the spring effect and the parametric amplification are observed, as well as optomechanically induced transparency and absorption. A device design for a microwave-to-optical transducer is developed, relying on the piezoelectricity of GaP. It combines electromechanical and optomechanical transduction. The predicted electromechanical coupling strength is in the MHz range. Furthermore, photonic crystal cavity designs containing a slot at the center of the cavity are studied. According to simulations for slot widths below 30 nm, optomechanical coupling strengths g0/2pi>1 MHz could be achieved. Fabricated silicon photonic crystal cavities show high quality factors of Qo=8x10^4 while hosting a mechanical eigenmode with a frequency of 2.7 GHz. Because of process technology limitations, only slot widths as narrow as 40 nm can be fabricated, the achieved g0/2pi is limited to 300 kHz. The new GaP-on-insulator material platform opens the door to integrated GaP devices. Frequency combs are of interest for soliton comb formation, mid-IR frequency combs, and ultra-broadband supercontinuum generation. Microwave-to-optical transducers are on the one hand desired for quantum information processing, on the other hand they are applicable as efficient modulators or detectors for classical signals.

EPFL2019

Chargement

Second-harmonic generation with free-running Nd:YAG laser

Sébastian Favre

SPIE2003

Chargement

Industrial laser precision machining is predominately based on pulsed Nd:YAG lasers operated at a wavelength of 1.06 µm in a free running pulse regime. The objective of this thesis work has been to investigate second and third harmonic generation for such "long pulse" Nd:YAG lasers. Visible and UV radiation can be better focused. The absorption coefficient at these wavelengths is higher in most materials. Processing at the doubled or tripled frequency increases machining precision and allows processing of new classes of materials such as, e.g., high reflective metals or ceramics. Frequency doubling and tripling of Q-switched, modelocked, or CW operated Nd:YAG lasers has been the subject of numerous publications. However, the particular problems associated with the free running pulse regime have not yet been addressed. The generation of harmonics requires a laser source that can deliver radiation with high brightness. A system based on a slab crystal with a beam quality factor of M2 ≈ 1.6 and a power of up to 1.5 kW has been used in the experiments. An external electro-optical switch has been implemented in order to precisely control the laser pulse shape and duration. The non-linear coefficient and the angular acceptance have been the main criteria for selecting LBO (LiB3O5), KTP (KTiOPO4), and MgO:LiNbO3 for the frequency conversion experiments. The conversion efficiency has been measured for these crystals as a function of the intensity at the fundamental wavelength. In contrast to simple model predictions, the efficiency does not increase linearly with intensity under long pulse conditions. An intensity dependant phase mismatch is at the origin of the deviations. A heuristic model for the efficiency prediction has been developed. It is based on a linear increase of phase mismatch with intensity and it allows describing the experimental findings with sufficient accuracy. The conversion efficiency for frequency doubling is limited to about 20% at pulse durations of 100 µsec. It decreases with increasing pulse duration. Catastrophic thermal damage is at the origin of the limitation in this pulse regime. Our experiments indicate that the damage is initiated by the frequency doubled light. Temporary color centers are created in the presence of the second harmonic light. The dynamic of the creation and annihilation of these centers has been investigated. It could be shown that finally the absorption of the fundamental harmonic (infrared) radiation at these centers results in irreversible thermal damages. A power of 140 W in the second harmonic (178 MW/cm2 on a waist of 5 µm) and 12 W in the third harmonic (61 MW/cm2 in a waist of 2.5 µm) was obtained with pulses of 200 µs. This allowed to demonstrate the feasibility of material processing with very high reproducibility and definition, which for instance allowed the design and the manufacturing of micro-grippers and several other prototypes in the field of robotics and micro-engineering.

EPFL2001

Chargement

Integrated Gallium Phosphide Photonics

Katharina Schmeing

EPFL2019

EPFL2001